A rotary body, such as an image carrier, is provided in an image forming apparatus of an electro-photographic system. Various mechanisms for transmitting driving force to the rotary body in order to rotate the rotary body at constant velocity have been proposed. For example, JP-A-9-90853 discloses forming four elongated holes extending in a radial direction at every 90 degrees in a disc-like member. The disc-like member rotates integrally with a driving gear, while four pins provided on a photosensitive drum are engaged with the elongated holes, thereby canceling a force applied to the photosensitive drum in the radial direction.
However, although the above mechanism can perform accurate constant-angular-speed transmission, at least in principle, it has a complicated structure and is expensive. Since the disc-like member intervenes between the driving gear and the photosensitive drum, and the four pins are slidably engaged with the four elongated holes, backlashes (gaps) are accumulated at four positions between the driving gear and the photosensitive drum. Further, because plural members, such as the pins, elongated holes and disc-like member, are interposed between the driving gear and the photosensitive drum, driving may become inaccurate due to elastic deformation, inclination, or the like of the plural members. This problem can be solved by making the members accurate to decrease the backlashes, and by employing metal instead of resin as material of the members. However, the cost further increases.
Aspects of this invention provide a driving force transmitting mechanism for a rotary body, which is manufactured at low cost and easily rotates the rotary body, such as a photosensitive drum, at constant velocity.